Hackers are cyber-attacking three Greek banks and demanding ransoms in bitcoin

REUTERS/Kacper Pempel ATHENS (Reuters) - Hackers have staged cyber-attacks on three Greek banks and demanded a ransom in bitcoins, a virtual monetary unit, to stop their disruption, banking sources said on Monday.

The sources said the hackers managed to block the Internet banking activity of three Greek lenders for a few hours last Thursday but did not penetrate the banks' security or obtain confidential client data or access to accounts.

"All they achieved was to block the web banking for a few hours. Nothing else," one banker told Reuters, speaking on condition of anonymity. The sources said the hackers had given the name of their group as Armada Collective.

The banks had refused to pay up and alerted the security services and the Greek central bank, which are investigating.

"We informed the police and the country's secret services are involved," a second banker said. "It's an easy to handle situation. There is no need for bank clients to worry."

A hacking extortion group using the same name was reported in mid-November to have staged sustained Distributed Denial of Service attacks on several private email services.

The attacks work by overloading the websites until they are forced to inhibit access or go offline.